This invention relates generally to a high intensity discharge (HID) lamp ballast and, more particularly, to an ignition scheme for starting an HID lamp.
An HID lamp generally refers to a family of lamps including high pressure mercury, high pressure sodium, metal halide and low pressure sodium. A conventional ballast for powering an HID lamp includes an ignitor. The ignitor provides high voltage ignition pulses applied to the lamp for starting the latter.
The energy from the high voltage pulses generated by the ignitor are provided to the lamp prior to the lamp entering its glow mode. At the time that the lamp begins to enter its glow mode, the ignitor is shut off. No high voltage pulses are generated during the glow mode. When ignition is successful, the lamp proceeds rapidly through the glow mode to a steady state condition, that is, from a glow discharge to an arc discharge condition between the lamp's electrodes.
The amount of energy required for successful ignition varies and depends on factors such as, but not limited to, partial pressures of the gasses within the lamp. When an insufficient amount of energy is delivered to the HID lamp to ignite the latter, the lamp can become stuck in its glow mode and never reach a steady state (full arc) condition.
Repeated exposure to prolonged periods within the glow mode can damage the HID lamp electrodes. Destruction of the lamp can result.
Accordingly, it is desirable to provide an improved HID ballast having a more reliable HID ignition scheme. The ignition scheme, in particular, should avoid prolonged periods of time within the glow mode.